1. Field of the Invention
Antilock brake systems ("ABS") have been developed to reduce the stopping distance and improve the stability of vehicles during braking. In these systems, the braking force at a wheel is modulated when the wheel is about to lock up, so that the wheel may spin freely and engage the surface fictionally again. This modulation is performed each time the wheel is about to lock, thereby constituting an ABS control cycle. A full antilock brake stop ordinarily consists of several such ABS control cycles.
To optimize braking performance, some of these systems attempt to modulate the braking force as a function of some of the braking conditions encountered. For example, more braking force may be applied to a wheel travelling on a surface having a relatively high coefficient of friction, such as dry pavement, than to a wheel travelling on a surface having a relatively low coefficient of friction, such as ice. Many braking conditions, however, can vary significantly from stop to stop, and even from ABS control cycle to ABS control cycle in a given stop. Furthermore, while some of these varying conditions (such as surface conditions) can be assessed and compensated for directly, others cannot be determined easily, or cannot be determined at all. These unknown parameters, if not compensated for, can greatly compromise the effectiveness of the antilock brake system.
Accordingly, the present invention generally relates to an antilock brake system that incorporates an adaptive control feature to compensate for these unknown parameters. More specifically, this invention relates to an antilock brake system that modulates the brake pressure at a slipping wheel in one ABS cycle as a function of the behavior of that wheel in previous cycles to correct for deviations from optimal performance, and thereby to reduce the stopping distance and improve the stability of the vehicle during the stop.
2. Description of the Prior Art Antilock brake systems usually include sensors for determining the rotary speed of at least two of the wheels of the vehicle. These wheels speeds are compared with a calculated vehicle reference speed. If during braking the rotational speed of a wheel is determined to be less than the calculated vehicle speed by more than some threshold amount, called the "slip threshold", that wheel is determined to be "slipping" or "exhibiting a tendency to lock up." The ABS then modulates the brake force, usually supplied by pressurized fluid like hydraulic fluid, applied to the wheel so as to permit the wheel again to engage the road surface fictionally and recover its speed. This process is repeated by the ABS, thus periodically "pumping" the brake to bring the vehicle to a stop.
(It should be noted that other schemes are known for determining that a wheel is exhibiting a tendency to lock. For example, rather than comparing the wheel speed with a calculated vehicle reference speed, it is known to measure wheel deceleration as an equivalent indicator of impending lock-up. Nevertheless, the present invention is not limited to any particular scheme for determining wheel lock-up.
In addition, the terms "brake force" and "brake pressure" will be used generically in this specification to refer to hydraulic brake pressure and other force applying mechanisms.)
In conventional antilock brake systems, the braking force at a wheel that exhibits a tendency to lock is typically first reduced to allow the wheel to accelerate back up toward the vehicle speed (or "spin-up"), then is held constant, and finally is increased at a controlled rate. In some of these systems, the rate of pressure increase is varied to account for surface conditions. For example, British Patent No. 1,251,892 (assigned to Messier) relates to a brake control system that determines the coefficient of friction between the wheel and the road by assessing the acceleration of the wheel as it spins-up. If the wheel acceleration is high, the system determines that the coefficient of friction is relatively high, and accordingly increases the brake pressure at a rapid rate. On the other hand, if the acceleration is low, the system determines that the coefficient of friction is low, and increases the brake pressure more gradually.
However, there are many other vehicle and wheel parameters that vary significantly over time. Many of these parameters, such as, for example, tire wear, tire pressure, brake wear, vehicle loading and master cylinder pressure, are not readily observable by sensors on the vehicle, and are therefore not commonly available as inputs to an antilock brake control system. These parameters, while not directly observable, all affect the performance of the antilock brake control system. Thus, it would be desirable to have an antilock brake system that could compensate for these parametric variations without directly measuring them.
U.S. Pat. No. 4,054,328 (Leiber et al.) relates to an antilock brake control system that uses adaptive control (or "feedback") in an attempt to modulate ideally the rate at which brake pressure is reapplied to a wheel that has recovered from an impending locking condition. In accordance with this system, brake pressure is reapplied to the wheel in a steep pressure increase period followed by a more gradual pressure increase period, with the duration of the steep pressure period in one ABS control cycle being varied in dependence on the duration of the gradual pressure increase period in the immediately preceding ABS cycle. Hence, this system purports to achieve optimal control of the brake pressure by measuring how long pressure was applied to the wheel in one control cycle, and by using that measurement to determine how the pressure will be applied to the wheel in the following control cycle.
Prior art systems such as those described in the patents mentioned above do not provide entirely satisfactory results, because they do not use parameters that give accurate indications of the past behavior of the wheel. Therefore, there is a need to develop a substantially improved control scheme for ABS that departs from past, outmoded approaches.